John 5:46-47 "For if you believed Moses, you would believe Me; for he wrote about Me. But if you do not believe his writings, how will you believe My words?"
God loves the Israelites but unfortunately only godly and devout Jews will believe Jesus as their Messiah. The Jews then were hypocrites, merciless and faithless and were destined to damnation.
Luke 16:31 "But he said to him, 'If they do not hear Moses and the prophets, neither will they be persuaded though one rise from the dead.'"
John's Baptism of Repentance
Luke 1:13,16-17 But the angel said to him, "Do not be afraid, Zacharias, for your prayer is heard; and your wife Elizabeth will bear you a son, and you shall call his name John... And he will turn many of the children of Israel to the Lord their God. He will also go before Him in the spirit and power of Elijah, 'to turn the hearts of the fathers to the children,' and the disobedient to the wisdom of the just, to make ready a people prepared for the Lord."
Just before the coming of their Messiah, the Jewish leadership and Priesthood then were without justice, mercy and faith and in Matthew chapter 23 our Lord Jesus condemned them as hypocrites. The Jews too were covetous and wicked. They cheated and robbed God in tithes and offerings as prophesied in the book of Malachi. (Same scenario with Christians before Rapture)
So John the Baptist (the prophet Elijah) was sent by God was to bring the Israelites back to Him by their repentance before God made the Old Covenant obsolete at the crucifixion of their Messiah (the great and dreadful day of the Lord).
Luke 3:3-4, 6-7, 11 And he went into all the region around the Jordan, preaching a baptism of repentance for the remission of sins, as it is written in the book of the words of Isaiah the prophet, saying: "The voice of one crying in the wilderness: 'Prepare the way of the Lord; Make His paths straight... And all flesh shall see the salvation of God.'" Then he said to the multitudes that came out to be baptized by him, "Brood of vipers! Who warned you to flee from the wrath to come?... He answered and said to them, "He who has two tunics, let him give to him who has none; and he who has food, let him do likewise."
Mat 3:1-2, 5-6 In those days John the Baptist came preaching in the wilderness of Judea, and saying, "Repent, for the kingdom of heaven is at hand!"... Then Jerusalem, all Judea, and all the region around the Jordan went out to him and were baptized by him in the Jordan, confessing their sins.
The Jews were told to repent for their Messiah would come soon to establish His kingdom on earth.
Note after His resurrection, God establishes His kingdom on earth in the physical body of every believer.
Luke 17:20b-21 He answered them and said, "The kingdom of God does not come with observation; nor will they say, 'See here!' or 'See there!' For indeed, the kingdom of God is within you."
Jesus preached Kingdom's Gospel
Mark 1:14-15 Now after John was put in prison, Jesus came to Galilee, preaching the gospel of the kingdom of God, and saying, "The time is fulfilled, and the kingdom of God is at hand. Repent, and believe in the gospel."
After John was imprisoned, Jesus and His disciples preached the gospel of the kingdom to get the Jews to repent and come back to believe Moses.
Luke 9:1-2 Then He called His twelve disciples together and gave them power and authority over all demons, and to cure diseases. He sent them to preach the kingdom of God and to heal the sick.